Conventionally, when saving a document as an electronic document generally a scanner, a camera, or the scanner section of a facsimile machine is used as an image input apparatus. In this type of scanner (scanner section), a light source is provided inside the apparatus and light emitted from the light source which is reflected from the document is read by a CCD or the like. If necessary, the read image is then binarized and saved. Because the characteristics of the used light source and optical system are constant, shadows and unevenness generated in the brightness are also constant and, therefore, can easily be corrected. Accordingly, high quality images can be output and digital images can be binarized easily and at high quality.
On the other hand, in recent years, with the rise in popularity of video cameras and digital cameras, there has been an increasing demand to be able to recognize written characters in images input from these types of devices. With regard to digital cameras, in particular, as the number of pixels have increased markedly and their size reduced, they have begun to be used in various applications as portable information acquisition tools. For example, character information such as documents, notice boards, and advertisements provide sufficient information as binary images and, moreover, the amount of storage space required to save this information is less than multi-valued images so that it is advantageous to save the information as a binarized image. Furthermore, binarized images can be transmitted using a facsimile or reused after undergoing a character recognition process.
Japanese Patent Application Laid-Open No. 3-237571 “Device For Calculating Binarized Threshold of Image” discloses a technology of suitably binarizing these types of digital images. According to this technology a high quality binarized image is obtained by providing a binarization circuit which performs binarization processing on an image in a window by comparing the difference between the brightness of each pixel in the window and the brightness of a specific pixel with a parameter which is proportional to the contrast of the observed image portion and by providing a determining circuit which determines the suitability of the binary pattern obtained by the binarization circuit as an image pattern of a contour portion.
Japanese Patent Application Laid-Open No. 7-212591 “Image Binarizing Device” also discloses a technology of binarization. According to this technology a histogram of luminance values is generated from a multi-valued image and the white pixel representative values and the black pixel representative values determined from the histogram. A binarization threshold value is then determined from the mean thereof, and the multi-valued image is binarized on the basis of the binarization threshold value. This patent application also discloses a technology in which an image is divided into blocks, the binarization threshold value or white pixel representative value/black pixel representative value for each block is determined, blocks with no characters therein are interpolated from surrounding blocks, and block threshold values are adopted as the threshold values for each pixel, thereby allowing brightness unevenness and shadows in the image to be removed. Thus, high quality binarization can be performed using the technology disclosed in this patent application.
However, the problems described below exist in the conventional technology. Namely, the number, position, and strength of the light sources are different, and therefore shadows and brightness unevenness easily occur in the photographed image. Moreover, because the shadows and brightness unevenness are inconstant, it is not possible to apply uniform correction as with the scanner section of a photocopier machine or the like. Accordingly, images photographed using a digital camera have the problem that high quality binarization is not possible.
Moreover, even if images have been photographed using a homogeneous light source, peripheral light reduction is caused by the properties of the optical system resulting in the problem arising that uniform correction and high quality binarization are not achievable.
Furthermore, for example, on a notice board, some characters or a group of characters are of small size. If such a notice board is photographed then there is a need to perform partial high quality binarization.
In Japanese Patent Application Laid-Open No. 3-237571 “Device For Calculating Binarized Threshold of Image”, it is required to provide the binarization circuit for performing image processing on each pixel in a window, and the determination circuit for performing pattern matching on the contour portions as an image pattern. Accordingly, there is the problem that the processing load has substantial increased and the processing speed has consequently decreased.
Moreover, in Japanese Patent Application Laid-Open No. 7-212591 “Image Binarizing Device”, in addition to generating a luminance value histogram for all the pixels, it is necessary to determine the white and black pixel representative values using loop processing and perform edge enhancement processing to enhance the edges of characters and diagrams. Accordingly, there is the problems that considerable processing resources are required and the rate of power consumption is also high.